1. Field of the Invention
The present invention relates generally to timer circuits, and more particularly to a timer circuit that clocks a predetermined time by dividing a frequency of an oscillating signal output from an oscillator.
2. Description of the Related Art
A charge controller is connected between a direct-current power supply such as an AC adapter or a USB port and a secondary battery such as a lithium-ion battery, and controls an operation of charging the secondary battery.
FIG. 3 is a block diagram of an example of a conventional charge controller 1. The charge controller 1 is configured of a semiconductor integrated circuit, and is connected between a direct-current power supply 2 and a secondary battery 3. In the charge controller 1, a charge control unit 4 controls a charge current flowing through a transistor 5. A timer circuit 6 clocks the time from when charging starts. For example, when the voltage of the secondary battery 3 does not exceed a predetermined voltage and charging is uncompleted even after the passage of a predetermined time such as four hours, it is determined that the secondary battery 3 is abnormal (e.g., deteriorated). Accordingly, the charge control unit 4 stops the charging operation.
Patent document 1 discloses a technology in which a switch is provided between a secondary battery and a constant-voltage circuit. Voltage between a constant-current circuit and the constant-voltage circuit is monitored. When the monitored voltage reaches an open-circuit voltage of the constant-voltage circuit, a control circuit stops the charging operation.
Patent document 1: Japanese Laid-Open Patent Application No. H11-265734
Conventionally, when checking operations of a charge controller in a manufacturing process, it takes a long time to check the operations of the timer circuit 6. A conceivable measure for reducing the time required for checking operations of the timer circuit 6 is to accelerate clock signals used by the timer circuit 6, i.e., increase the frequency of the clock signals.
In order to accelerate the clock signals, an element such as a condenser can be externally attached to an external terminal of the semiconductor integrated circuit functioning as the charge controller. The attached element is used for determining the time constant of an oscillator that generates the clock signals. When the charge controller is used under normal conditions, the element (condenser) used for checking operations of the charge controller can be replaced with another element. However, in order to provide such an element, the external terminal needs to be added to the semiconductor integrated circuit. When circumstances do not allow for an external terminal to be added to the semiconductor integrated circuit, this measure cannot be realized.